1. Field of the invention
The present invention concerns an optical waveguide made of solid state material, a laser applying this waveguide and a method for carrying out the same.
2. Summary of the prior art
Solid state materials have provoked an increasing amount of interest over the past fifteen years for their noteworthy properties in optical telecommunications as light sources and detectors in the spectral field close to the infrared, in particular where the silicon optical fibers present very low attenuations and dispersions. Alongside studies aimed at rendering end components such as laser diodes and photodiodes more reliable, and offering improved performances, the possibility of better processing data by means of the integrated optics has been adopted. First of all, on passive substrates (glass), then active substrates (lithium niobate or tantalate) phase and amplitude modulating components, frequency translators or light commutators in guided medium have produced and have demonstrated the novel potential of a processing operation in the optical form of data. It is only recently that research into integrated optics on solid state materials III.V especially have allowed to envisage the integration upon a single substrate of sources, detectors, modulator components and commutators.
In the field of laser diodes, it is necessary to obtain closely controlled laser diodes and power laser diodes, even laser grid diodes. To do this, the base of a laser diode lying in a waveguide, the purpose must thus be to optimize this waveguide in order to obtain greater precision and higher power values.
It is consequently necessary to dispose of waveguides having very low losses in the solid state materials III.V produced in such a manner that it is compatible with technology currently adapted for laser diodes. Therefore, the maximum benefit of novel techniques of epitaxy and selective chemical attack of thin or ultra-thin layers (several tens of Angstroms in the case of quantic wells). For example, the Low Pressure Metallorganic Chemical Vapor Deposition LP MOCVD epitaxy technology or molecular jets epitaxy MBE can be used. In order to understand the guiding method in solid state devices, it is possible to utilize, among other methods, the effective indice methods which will briefly be recalled herein-below. The guiding structure represented in FIG. 1 can be divided into three juxtapositioned plane guides each defined by a series of optogeometrical parameters. It is therefore possible to determine analytically the propagation constants of each of the plane guides taken separately when they are formed of constant index mediums n.sub.i, which will always be the case for those structures currently envisaged. These propagation constants .beta..sub.i can be conceived as refraction indices of another plane guide of which it is also possible to calculate the propagation constant .beta.. The method of effective index consists in F considering as the propagation constant of the initial structure. This method has a physical interpretation in theory of the radii when these latter are reflected at the front of different indice mediums. Everything happens therefore as if the light was confined by a medium of width w having an index .beta..sub.2 and surrounded index semi infinite mediums having an index .beta..sub.1 such as represented on FIG. 2.
The most simple structures that allowed to obtain the first guides in solid state or semiconductor materials have been constituted by the deposition of a thin slightly doped guiding layer on a substrate of several microns of thickness considered as infinite at the wavelengths of the infrared utilized. Such a structure is represented at FIG. 3, with the guide layer referenced 1 and the substrate referenced 2. In this case, the structure is surrounded by an upper optical confining layer which is generally air (with an index n=1) and the parameters w, h1, h2 are adjusted while being aware of the refraction indexes n1 of the guiding layer and n2 of the substrate in order to obtain the desired guiding by effective index. These structures generally present considerable losses for two reasons. First, the diffraction through the irregularities of the vertical sides of the guide can be very large due to the type of attack, on the one hand. Secondly, doped substrates are often utilized which allow to apply a control electrical field adjacent to the guide. The free carriers that are present are responsible for the attenuation of the evanescent part of the wave guided by these dielectric media.
More elaborate structures have thus been conceived and produced. They comprise optical isolating buffer layers of the guide of the strongly doped mediums. The rib type structure is thus produced by selective chemical attack of the upper buffer layer stopped on the material of the guiding layer. An example of such a structure is represented on FIG. 4. This structure comprises a substrate 21 bearing a n.sup.- doped buffer layer, which bears an optical guiding layer 1 completed by guiding elements 10 and 11 doped respectively n.sup.- and p.sup.+, and which excited by an electrical field applied to a metallic electrode 12 allows a phase modulation of a luminous wave through the electrooptical effect.
By way of example, the substrate 21 and the buffer layer 20 can be made of indium phosphide (InP). The guiding layer and the guiding elements can be made of gallium and indium arsenide (GaInAs) or of Gallium and Indium arsenide phosphide (GaInAsP).
Potentially better, this structure allows with difficulty to reach losses lower than 1 or 2 dB/cm (in InP/InGaAsP at .lambda.=1.55 .mu.m). If the upper confinement material of the structure is constituted by air, the guiding by effective index is strong and can raise a problem if it is desired to obtain a directional coupling since it is then necessary to move the rib guides closer to distances sometimes smaller than one micron. To reduce the guiding by effective index, it is possible to leave a part of the upper buffer layer, but this is generally of about 1 micron thickness and technologically it is very difficult to obtain surface conditions minimizing the losses through diffusion. Furthermore, the benefit of the property of the surface condition that is obtained in selective chemical attack stopping on an epitaxied layer by MOCVD method in vapor phase having an excellent optical quality is lost.
This is why the invention concerns a guide structure that can be produced through associating the vapor phase epitaxy method and the selective chemical attack method.
The vapor phase epitaxy method allows to obtain ultra thin layers allowing to reduce as desired the guiding by effective index. The selective chemical attack method allows to tool patterns on one layer without modifying the others.
The structure thus obtained is produced with a high precision on the one hand relating to the thickness of the layers and on the other hand relating to the geometry of these elements and it furthermore allows to obtain an excellent surface quality.